Well logging systems



6 Sheets-Sheet 1 B A c W. P. SCHNEIDER WELL LOGGING SYSTEMS June l1, 1963 Filed Nov. 29. 1961 6 Sheets-Sheet 2 Filed NOV. 29, 1961 WMM@ ATTORNEY June l1, 1963 w. P. SCHNEIDER WELL LOGGING SYSTEMS 6 Sheets-Sheet 5 Filed Nov. 29, 1961 9 w cx w Q Q wk Q ...5? .v #$9.1 wk Y n G, T G, Q Y e u Y w Q AT Qmw mk SSQ N hwwx NQQ E L L Ih-v l .Gu, d d buh); l l muw, .w xvw wl Tl hwq QN K K E QN N NN K K u huw! I. I. Il I .I .Il I Il I l i I l l l I vw .k K l i l l l @hw k VV////0/77 /D JcbnQ/a/e June M, 1963 w. P. SCHNEIDER WELL LOGGING SYSTEMS 6 Sheets-Sheet 4 Filed Nov. 29, 1961 www1..

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6 Sheets-Sheet 5 Filed NOV. 29, 1961 June ll, 1963 w. P. SCHNEIDER WELL LOGGING SYSTEMS 6 Sheets-Sheet 6 Filed Nov. 29, 1961 PMX PQR ww wk Q w mv L Q IILK w L E E IFLIHL .N lfLJL ATTORNEY Patented .lune ll, l63

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ntlglt WELL LGGENG SYSTEMS William P. Schneider, Houston, Tex., assigner to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Terras Filed Nov. 29, i951, Ser. No. 55,675 Claims. (Qi. 34th-i3) This invention relates to well logging systems and, more particularly, to acoustic well logging systems for obtaining measurements of acoustic parameters of a well bore through which the tool may be passed.

In the acoustic'logging of well bores, considerable efforts have been devoted to the measurement of the time required for an acoustic impulse to travel over a short vertical section of a well bore and successive time measurements for such short sections are recorded as a function of depth. The derived time measurements are related to the characteristic velocity of the media forming the well bore, as well as the porosity of the media. Obviously, the accuracy of the time measurements is of eX- treme importance in providing accurately interpretable results.

When one considers that the time measurements are obtained from a borehole instrument disposed in a well bore which may have temperature conditions of up to 375 F. or more and pressures of 20,000 p.s.i. or more and that electrical signals from the borehole instrument must be transmitted to the earths surface over a multiconductor cable which not only must provide a faithful ransmission path but also supports the weight of the borehole instrument and is also subjected to well bore temperatures and pressures, the problems encountered in deriving an accurate time measurement are seen to be extremely complex. These problems may be neatly classied in three groups, ie., the borehole instrument, the transmission and structural characteristics of the cable and the surface instrumentation.

lt is necessary that the time measurements be reliable representations of the time required for acoustic energy to travel through the media forming the well bore to the exclusion of the time required for the acoustic energy to travel between the instrument and the media. Thus, in a borehole instrument where a single transmitter and receiver are used (hereinafter sometimes referred to as a T-R system), it is necessary to correct a time measurement value by an increment of time representing time of travel of acoustic energy between the instrument and adjacent media. Since this increment of time is sometimes subject to error due to irregular spacing of the instrument from the media, a single transmitter and two receiver system (hereinafter sometimes referred to as a T-R-R system) has been developed. In a T-R-R system, the time of arrival of an acoustic impulse at successive receivers is detected and the time interval between such detections measured. Thus, increment of time corrections for acoustic energy travelling between the instrument and the well bore are eliminated. However, even in this latter system, errors in time measurement arise where the instrument tilts relative to the axis of the well bore or the well bore surface is irregular due to changes in the diameter of the ell bore. Thus, a borehole instrument with a T-l?. or T-R-R system inherently can be the cause of inaccuracy in time measurements because of positioning relative to the wall of the well bore. Or, in other words, the accuracy of these systems depends largely upon a uniform spacing of the transmitters and receiver from the wall of the well bore.

Considering now the signals detected in a borehole instrument, the detected signals may represent the time of emission of an acoustic impulse and/ or represent `acoustic energy as sensed by one or more receivers. The detected signals may be either transmitted via the cable to surface located time measuring means or supplied to time measuring means in the borehole instrument. In the latter instance, a signal representing a time measurement is conveyed to surface located indicating means via the cable.

Where the time measuring means are in the borehole instrument, a time measurement derived by such means is generally represented by a voltage signal. The downhole time measurement system is generally complex and expensive due to the instrumentation necessary to withstand the borehole temperature and pressure conditions, space limitations, shock, etc., and yet provide a reliable time measurement signalV under the varying well bore condi-V tions. However, this system can provide an analog signal representing a relatively short time measurement of 40 microseconds to 200 microseconds for a one foot span between receivers in a T-R-R system. An analog signal is transmitted through the Cable to surface recording instruments.

To pass the detected signals as sensed by the receivers in the borehole instrument directly to the earths surface has heretofore been undesirable because of the short time duration between detected signals and the fact that the cable affects the character of the detected signal. Distortions of the signal introduce errors in the time measurements at the earths surface.

The faithful transmission of signals as detected in a borehole instrument to the surface instruments via a well logging cable with fidelity, however, has now been made possible by the use of an armored seven conductor cable in which six conductors are spirally embedded about a central, seventh conductor with equalizing means being provided for signal transmission purposes. The seventh conductor has been found ideally suited for the transmission of a signal with fidelity. Such a cable is described in a co-pending application assigned to the assignee of the present invention and is identified as patent application Serial NO. 79,390, tiled December 29, 1960.

The beneficial transmission characteristics are ascribed mainly to the uniform spacing of the armor wires about the central conductor where the armor wires provide a1 electrical ground. By analogy, therefore, a coaxial type of cable would also be suitable for the transmission of a signal with fidelity.

Nonetheless, even though signals from the borehole instrument can now be transmitted via the cable to the earths surface with fidelity, accurate time measurements of the travel of acoustic energy along the media forming the well bore .are dependent upon the relative position of the borehole instrument in the well bore and accuracy of time measurement in the time measuring means.

Accordingly, it is an object of the present invention to provide new and improved acoustic logging systems in which the accuracy of time measurements at the earths surface is markedly improved.

Another object of the present invention is to provide new and improved acoustic logging systems in which detection of acoustic signals is more accurately determined in the borehole independent of the geometry of the well bore relative to the borehole instrument.

Another object of the present invention is to provide new and improved acoustic logging systems with overall increased accuracy in measuring intervals of time.

rThe acoustic logging system in accordance with the present invention includes a borehole instrument comprised of an upper transmitter, an upper receiver, a lower receiver and a lower transmitter which are operated to obtain independent signals representing acoustic energy traversing the media forming 'the well bore from above and below the receivers and providing at least two time measurements which are averaged to provide an average travel time. Means at the earths surface are provided to accurately determine time intervals between the emissions of acoustic impulses and arrival thereof at different receivers from a sequence of signals developed by the borehole instrument, the means at the earths surface providing an ultimate averaged indication based upon acoustic energy emissions from both transmitters and acoustic arrivals at the respective receivers.

The novel features of the present invention are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation together with further objects and advantages thereof, may best be understood by way of illustration and example of certain embodiments when taken in conjunction with the accompanying drawings in which:

FIG. l is a view of a borehole instrument suspended in a well bore and coupled to circuitry at the earths surface by a cable;

FIG. 2 is a schematic representation of the circuitry located at the earths surface;

FIG. 3 is a schematic representation of circuit-ry in the borehole instrument;

FIG. 4 is a view of typical waveforms particularly with reference to the operation between the surface equipment and borehole equipment;

FIG. 5 is a view of typical waveforms particularly With reference to the operation of the surface equipment of FIG. 2; a n

FIGS. 6a and 6b are diagrammatic views of surface located time measuring means;

FIGS. 7-9 are simplified representations of various timing sequences of the system; and

FIG. l0 is a schematic representation of counter and transfer circuits of the present invention.

Referring now to FIG. 1, an acoustic Ilogging tool or apparatus 10 is shown disposed in a borehole 11 by means of a cable 12. which is spooled on a reel or winch *13 located at the earths surface. The apparatus 'l0 is preferably centered in the well bore by conventional centering means (not shown).

The electrical circuitry in the tool 10 is coupled by the cable `l2 to surface located electrical circuitry 14. The tool It) includes an array of transducers which may, for example, be of the magnetostrictive type and are suitably supported in a fixed spaced relationship to one another in a Well-known manner in the tool. The transducer arrangement provided includes an upper transmitter T1, an upper receiver R1, a lower receiver R2 and a lower transmitter T2 in longitudinal alignment with the spacing between the transmitter T1 and receiver R1 equal to the spacing between the transmitter T2 and receiver R2. Preferably, this spacing is on the order of three feet and the span between receivers R1 and R2 is ion the order of one foot. The transmitter T1 and receiver R1 are thus symmertical relative to the transmitter T2 and receiver R2 about a plane of symmetry midway between receivers R1 and R2.

The operational arrangement of the T-R-R-T system is such that the time of emission of a pulse of acoustic energy from a transmitter can be reliably detected at the earths surface and the acoustic energy as detected by a receiver can be representatively reproduced as an electrical signal at the earths surface and the travel time of acoustic energy from the transmitter and through the adjacent media forming the well bore and back to a receiver can be measured with considerable accuracy. At the earths surface, the transmitter-to-receiver signals are received by the circuitry 14 from the down hole tool 10l in a sequence in which one Atransmitter is pulsed twice to provide emission signals alternating with signals respectively `detected by receivers spaced a long and a short distance from the first transmitter; the other transmitter is also pulsed twice to provide emission signals alternating with signals respectively detected by receivers spaced a long and a short distance from the second transmitter.

A pair of the sequenced signals thus represents the time interval bet-Ween the emission of an acoustic pulse and its arrival at a given receiver. In the circuitry 14, a irst time interval between the emission of an acoustic pulse and i-ts arrival at a given receiver spaced the long distance lfrom the rst transmitter is stored in a counter circuit. A second time interval between the next succeeding acoustic emission of the first transmitter and the arrival of acoustic energy at the receiver spaced the short distance from the first transmitter is then subtracted from the iirst time interval. The next time interval between the emission of an acoustic impulse by the second transmitter and its arrival at a receiver spaced the long distance from the transmitter is added into the counter circuit. The subsequent time interval between the emission of an acoustic impulse by the second transmitter and its arrival at a receiver spaced the short distance from the tra-nsmitter is subtracted from the counter circuit. Thus, the net time interval count left in the counter is representative of two distinct times of travel of acoustic energy over a section of adjacent media between the two receivers. The net time interval Vin the counter is divided by two thereby to provide an average travel time of acoustic energy over the section of adjacent media. One of the prime advantages of this system is that the longer time intervals measured between an emission and detection of acoustic energy permit time for accurate transmission lof the signals to the earths surface whereas with a short spacing between receivers it is diicult to transmit the signals directly to the earths surface. The transmitters above and below -the receivers further provide for time measurements substantially independent of the instrument position relative to the wall or geometry of the well bore.

Reference is now made to FIGS. 2 and 3 for the respecitive details :of the surface located electrical circuitry 14 and the electrical circuits in the apparatus 10. Also, reference will be made to FIGS. 4 and 5 wherein representative waveforms for various signals in the circuit are illustrated.

As shown in FIG. 2, the repetitive pulsing of a transmitter in the logging instrument may be controlled by a periodic pulsing source `20 such as a twenty-cycle per second signal generator whichy is arranged to develop a signal output 20a (FIG. 4a) to pulse the transmitter independent of the travel oit' the tool. On the other hand, the movement of the cable can be coupled by means of a sheeve 13a in a well-known manner to a selsyn system or interval timer Z1 to provide pulse signal outputs to the transmitter dependent upon the movement of the cable so that the pulsing of the transmitter is keyed to the movement of the tool through the well bore. A switch 22 is provided to connect either of these pulsing systems to a conventional flip-flop circuit 24 so that the output of a pulse source causes the flip-flop circuit 24 to reverse its operating condition. In FIG. 4b and FIG. 5a, a typical waveform 25a yon conductor 25 is illustrated, the waveform 26a on conductor 26 being reversed in polarity and shown in dashed line.

Outputs from the ilip-ilop circuit 24 with respect tot a ground potential are respectively conveyed by conductors 25, 26 to flip-flop circuits 27 and 28. A conductor output B is provided from the output of one side of flipflop 27 and likewise, a conductor output C is provided from the output of one side of ilip-ilop circuit 28. Conductor loutputs B and C are coupled via cable conductors to conductor inputs in the logging instrument (FIG. 3) which are correspondingly identiied as B and C. In FIG. 4c, the waveform 27a of the output signal on conductor output B is illustrated while the waveform 28a lof the output signal on conductor output C is illustrated in FIG. 4d. It should be appreciated that the output signals on conductor outputs B and C are out of phase.

Referring now to FlG. 3, the conductor input C is connec-ted to a solenoid 32 of a receiver switch 33. Thus, in one operating condition of the flip-hop circuit 28, the solenoid 32 is energized while in the other operating condition of the hip-flop circuit 2S, the solenoid 32 is deenergized. The solenoid actuated receiver switch 33 connects a common signal channel as indicated generally by the numeral 34, to first one receiver then the other. The conductor input B is connected to a solenoid 36 of a transmitter switch 37. Thus, in one operating condition of flip-flop circuit 27, the solenoid 36 is energized while in the other operating condition of flip-flop circuit 27, lthe solenoid 36 is de-energized. The solenoid actuated transmitter switch 37 connects a common transmitter channel as indicated generally by the numeral 33 to first one transmitter and then the other.

As shown in FiG. 3, the receivers R1 and R2 are respectively connected to the poles 41, 42 of the receiver switch 33, which has its movable contact arm connected to an amplifier i7 in the single signal channel 34. Receiver switch 33` as discussed above is automatically operated to change positions at the time that the flip-Hop circuit 28 reverses operating conditions. The transmitters T1 and T2 are respectively connected to the poles 43, 44 of the transmitter switch 37 which has its movable contact arm connected to the single transmitter channel 38. Transmitter switch 37 is actuated to change positions at the time that the iiipiop circuit 27 reverses operating conditions.

Returning now to FlG. 2, pulse selection switch 22 is also coupled by a conductor 23 to a conventional delay circuit 2.9. Delay circuit Z9 is in turn, connected to a tire pulse generator 30. The time delay of delay circuit 29 is made, for example, about 9 milliseconds to enable all of the down hole switching of switches 33 and 37 to be accomplished prior to the initiation of a fire pulse from pulse generator t. Pulse generator Btl is coupled by a conductor output A and the cable to a conductor input A of the transmitter channel 38 in the borehole instrument 16 (FIG. 3). The common transmitter channel 38, as shown in FIG. 3, includes a delay circuit 39 and a pulse generator 4@ which has its output coupled to the movable arm of the transmitter switch 37. When a fire pulse Etta (FIG. 4e) from pulse generator 30 is transmitted through the cable to transmission channel 38 via conductor input A, the pulse 39a is conveyed to the delay circuit 39. Pulse 39a is also conveyed via a conductor 47 to a iiip-iiop circuit d5. The delay circuit 39 has a time delay, of say, a l0() to 200 microseconds to permit the receiver channel 34 to be blocked before a transmitter is actuated, Thereafter the delay circuit 39 actuates the pulse generator 40 to develop a pulse output 40a (FIG. 4g) to trigger a transmitter which produces an acoustic impulse.

In the signal channel 34 (FIG. 3) the amplifier 47 is coupled to the movable arA i of switch 33 and has its output connected to a power amplifier 56. Noise gate 55 is normally closed (FIG. 4f) for a short time duration in response to the fire pulse a from the pulse generator 30 actuating flip-Flop 45 which controls the operation of gate 55. The time duration during which the noise gate 55 is closed is made long enough to block any signal from the receivers for transmission to the power amplifier 55 of the signal channel 34 prior to and shortly after the actuation of a transmitter.

The pulse output u (FIG. 4g) which triggers a transmitter also is supplied from the pulse generator 4t) via a conductor 59 to a pulse Shaper 46 which shapes the signal 40a indicating the time `occurrence of the firing of the transmitter. The signal Litta from pulse shaper 46 is conveyed to amplier 56 which is coupled via a capacitor 6i) to the central `or seventh conductor o1 of cable 12. The signal 49a from the pulse sh-aper `tl-6` is thus transmitted via the cable 12 by the mode of the seventh conductor and the cable armor fwhich is at electrical ground potential. Capacitor 60 is sized of a value in accordance with the electrical band width of response for the cable 12 to transmit the frequencies in the electrical signal corresponding to the acoustic energy as received by a receiver to the surface in a reliable manner. Thus, the transmission chanacteristics of a cable are equalized to insure a high fidelity signal at the surface of the earth.

At the earths surface as shown in FIG. 2, the cable conductor 6i from the single signal channel 34 in the borehole apparatus 10 is connected -to an amplifier 70 which, in turn, is connected to normally closed signal gate circuits 71 and 72. The transmitter signal gate 71 has its gate control circuit connected to a conventional delay circuit 73 which is, in turn, coupled by a condoctor 62 to the fire pulse generator 30. The fire pulse 39a (FIG. 5bk or FIG. 4e) actuates the delay circuit 73 which, after a suitable time delay 73a (FIG. 5c) has elapsed following the development of the fire pulse 30a, opens the gate 7i as shown in FIG. 5d. The time delay of circuit 73 is less `than the time delay of delay circuit 39 (FIG. 3) so that gate 71 is opened prior to the arrival of a pulse fitta indicating the triggering of a transmitter. After gate 71 is opened yor actuated, the pulse signal 40a conveyed via the amplifier I56 of the signal channel 34, the conductor 6l and amplifier 70 is passed through the gate 71 to a time of occurrence of signal detector 74 which develops an output pulse 74a (FIG. 5e). The time of occurrence detector 74 may be a suitable conventional triggering circuit arranged to produce an output trigger signal 74a (FIG. 5e) whenever the amplitude of an applied input signal from ampliiier '70 exceeds a predetermined level. Thus, the detector 7 4 is triggered upon receipt of the signal 40a from the pulse genenator 49 while the gate 71 is open. The output signal 74a of the detector 74 is returned to the gate 71 via a conductor 75 to close the gate 71 immediately after the ydetector 74 is actuated. The output signal '74a of the detector 74 is also supplied via a conductor 76a to a iiip-tiop circuit 76 which actuates a time or count gate 77 (FIG. 5h). Conductor 76a also is connected to a delay circuit 78 which, in response to signal 74a from the detector 74 and, after a suitable predetermined time (FIG. 5f), aotuutes or opens the remaining gate circuit 72 for operation (FIG. 5g). Normally, gate circuit 72 is opened by the delay circuit 78 just prior to the earliest expected arrival of an acoustic impulse at a receiver.

The timing gate 77, when turned on, permits the output signal (FIG. 5i) from Ia high frequency crystal oscillator 8) (for example, 5 megacycles for the described T-R-R-T preferred spacing) to be supplied to a counter circuit 31 which counts the pulse output from the crystal oscillator. Thereafter, a receiver signal supplied via amplifier 70 and cable conductor 61 to the receiver gate 72 is passed to a time of occurrence detector 82 which is similarly actuated upon the signal exceeding a predetermined amplitude level. The output signal 82a (FIG. 5j) of the detector 82 turns the timing gate 77 off (FIG. 5h) and also turns the receiver gate 72 oi (FIG. 5g).

During the time interval that the timing gate 77 is on or open, a number of pulses are supplied to the counter 31 which number is related to the time interval between an emitted transmitter pulse and la receiver signal. The counter circuit 81 is arranged to add or store the count representing a first time interval by virtue of lan add circuit 84 connected to conductor 25 of the iiip-op circuit 24. Thus, when the output signal 25a (FIG. 5a) of flip-flop circuit 24 operates nip-Hop circuit 27, the add circuit 84 is simultaneously actuated (FIG. 5k) to condition the counter 81 to add or store the pulse output from the crystal oscillator 30 during the time that timing gate 77 is open. On the next succeeding output signal 26u (shown in dashed line in FG. 5a) from the Hip-flop circuit 24, the flip-flop circuit 28 and a subtract circuit 85 are `actuated (FIG. 5l) to condition the counter S1 to subtract the output pulses of the crystal oscillator 80 from the previous retained count in the counter 81.

The arrangement operates in a sequence which may best be understood by a consideration of FIG. 7. At the time to, the waveforms on conductors outputs B and C and tlip-ilop 24 may be -assumed to be as shown in FIG. 7 with the resulting connection of transmitter T1 to the transmitter channel 38 and the connection of receiver R2 to the signal channel 34. Thus, as shown in FIGS. 4h and 4k, the transmitter and receiver signals from transmitter T1 and receiver R2 would cause timing gate 77 to be open for a time interval during which the pulse output of the crystal oscillator `80 is added into the counter oircuit 8.1. At the time t1 (shown in FIG. 7), flip-ilop circuit 24 actuates the subtract circuit 8S for the counter S11, transmitter switch 37 is unchanged and the receiver switch 33 is actuated. Thus, transmitter T1 is connected to channel 318 and receiver R1 fis connected to channel 34. The output of the crystal oscillator Si] in the time interval between lan emission of acoustic energy from the transmitter T1 (FIG. 4h) and its arrival at the receiver R1 (FIG. 4j) is subtracted from the count in the counter circuit 81 so that the count left in the counter 81 represents the time interval during which acoustic energy travelled the earth formations between the receivers R1 and R2. At the time t2 as shown in FIG. 7, flip-flop circuit 24 actuates the add circuit 84 for the counter, transmitter switch 37 is actuated and receiver switch T2 is connected to transmitter channel y38, and receiver R1 is connected to signal channel 34 since the receiver switch 33 remains unchanged. The output of the crystal oscillator 80 in 'the time interval between the emission of an acoustic impulse from transmitter T2 (FIG. 4i) and its 'arrival at the receiver R1 (FIG. 4j) is added in the counter and represents the time interval during which o acoustic energy travelled between the transmitter T2 and receiver R1. At the time t3 as shown in FIG. 7, flip-flop circuit 24 actuates the subtract circuit 85 for the counter, transmitter switch 37 remains unchanged and receiver switch 33 is actuated. Thus, transmitter T2 is connected to transmitter channel 38 and receiver R2 is connected to signal channel 34. The output of the crystal oscillator in the time interval between the emission of an acoustic impulse from transmitter T2 (FIG. 41') and its arrival 'at the receiver R2 (FIG. 4k) is subtracted from the count in the counter and represents the time interval during Iwhich acoustic energy travelled between the transmitter T2 and receiver R2.

From the foregoing description it will be :appreciated that if:

ta is the time of travel of acoustic energy between transmitter T1 and receiver R2;

tb is the time of .travel of acoustic transmitter T1 and receiver R1;

tc is the time of travel of acoustic `energy between transmitter T2 and receiver R1; and

td is the time of travel of acoustic energy between transmitter T2 and receiver R2;

Then At, the average time of travel of acoustic energy between receivers yR1 and R2 may be expressed as energy between Ag=gii (1) The average time of travel At is thus based upon acoustic energy emitted from above and below the receivers and is an average of two distinct time measurements. This reading minimizes errors due to tilt of the apparatus in the well bore or caves in the well bore and very greatly improves the accuracy of .the At time measurement.

To obtain a measurement of the average net count of the At time remaining in the counter 81, a transfer circuit S6 (FIG. 2) is connected to the counter 81 so as to -divide by two the net count in the counter. The transfer circuit 86 is actuated by rneans of .a signal 87a (FIG. m) from a delay circuit `87 which is coupled to conductor output C of lflip-flop circuit 28. Signal 87a Ifromdelay circuit 87 :actuates the transfer circuit 86 after the last time measurement and just prior to the ystart of a new cycle of the system. The transfer circuit 86 when actuated immediately reads out an average net count from the counter 81. The average net count as read by the transfer circuit 81 is translated by conventional binary to analog converter (BAC) 89 into a voltage signal supplied vto the conventional galvanometer 9) in a recorder (not shown) to produce an indication representative of the net time interval measured -by .the counter 81. Galvanometer 90 can, of course, be calibrated to indicate either time or velocity measurements in a well-known manner.

The transfer delay circuit 87 also has its output coupled to a reset delay circuit 87h which, after a suitable delay y87e (FIG. 5n), resets the counter y81 by means of a reset pulse 87e (FIG. 50) a short time after the net count in .the counter 8l bas been transferred to the transfer circuit 86 and just prior to the next tire pulse Sila so that the counter 81 is reset to measure the timing measurements of the next lsequential operation.

Referring now to FIGS. 6A, 6B and FIG. l0, the `details and voverall arrangement of the counter circuit 81, the subtraction circuits `R5, the addition circuits 84, and the transfer circuit S6, will now be explained. As Shown :in FIG. 10, the counter circuit 81 is basically comprised of a plurality of bistable multivibrator units 81(ak) coup-led to yone another to form a binary counting system. In the preferred embodiment of the invention, eleven such individual units' or stages are lconnected to lone another in which the units provide a digital ou-tput representative of the number of pulses applied to its input. For simplicity of illustration, in FIG. 6A only the rst, second, and part of the last unit of the counter 'are shown, it being understood that the intermediate units have similar arrangement to the system to be explained.

Each multivibrator of counter 181 has transistors 90, 91 cross-coupled to one another and responsive to 4a common input pulse to their respective bases to change operating positions. The input pulse signal from the gate 77 to the first multivibrator of the counter 81 is supplied to a common input connection 97 of an input circuit 96 comprised of a ydi-ode 94 and capaci-tance 95 in series' connection wtih each of conductors 92, 93, the capacitances terminating in the common input connection v97 and the conductors 92, 93 being connected to the respective bases of transistors 90 and 91. Hence, as the pulses of a pulse 'signal supplied via the gate circuit 77 are applied to the respective bases of the transistors 9i), 91 in the rst multivibrator 89, the multivibrator reverses operating conditions for each applied pulse of the pulse signal.

In the typical manner of counter circuits, in the first multivibrator 81a, the collector of the first transistor 90 provides an output signal which indicates either :a 0 or 1 digit `and the collector Iof the :second transistor 91 provides an output signal which indicates either a 0 or 1 digit. lIn the next adjoining multivibrator -811b and throughout the successive multivibrator units, the lirst transistor thereof would likewise provide an output signal which indicates either a 0 cr l digit and the second transistor thereof would provide an output signal which indicates either a 0 lor 1 digit. To add a number of successive pulse inputs in the binary `counter 811, the output of the second transistor 91 of each multivibrator is connected to :an input circuit 96 of the next adjacent multivibrator by means of output conductors 100 and isolation resistors 100e. To subtract a number of successive pulse inputs from a number of pulses which are stored in the binary counter by addition, the output of the rst transistor of each multivibrator unit is connected to an .input circuit 96a of the next adjacent multivibrator by means of output conductors 101 4and isolation 9 resistors ltila. Input circuits 96a are identical to input circuits 96 and are connected in parallel.

To perform the yaddition of separate and distinct pulse inputs, add circuits S4 are connected to the subtract output conductors itil. The add circuits 54 are provided with add switches 103 in the form of ltransistors which, when actuated by a `commonly applied pulse signal, short the subtract output conductors 161 to ground so that only the add outputs of second transistors 91 on conductors u are effective in operating the muitivibrators. To perform the subtraction of a pulse input, subtract circuits 85 are connected to the add output conductors 100 to adjacent multivibrator-s. The subtract circuits 85 are likewise provided with subtract switches 164 in the form of transistors which, when actuated by a commonly applied pulse signal, will short the add output conductors 100 to ground so that only the subtract outputs on conductors lill are effective in operating the multivibrator units. Thus, it will be appreciated that the add switch circuits 84, when actuated by a `common signal from the flip-flop circuit 2li, cause, the counter `S1 to add the pulse output from the crystal `oscillator during the time gate '77 is open. The time that gate 77 is open is determined by the time of pulsing of a transmitter and the subsequent arrival of the acoustic energy at a far receiver for example, between transmitter T1 and receiver R2. When the flip-Hop circuit 24 reverses its operating conditions, the subtract switch circuits 8S are actuated and the counter 81 is then conditioned to subtract the pulse output from the crystal oscillator during the time gate 77 is open. The time that gate 77 is `open is now determined by the time of pulsing `of a transmitter and the subsequent arrival of the acoustic energy at a near receiver, for examample, transmitter T1 and receiver R1. The net count of pulses are digitally stored in the counter circuit 81 `after addition and subtraction represents the net time for acoustic energy to travel the earth formations between the two receivers. Thus, the next addition and subtraction of pulses representing the travel time of acoustic energy between Tl--Rg and "T2- R2 respectively leave a net count in the counter of two net times for Vacoustic energy to travel the earth formations between two receivers where the net times are derived from pulsing of a transmitter above and below the detecting receivers.

The average net count in the counter circuit $1 is read Eout in the following manner. A transfer circuit 36 is provided which is comprised of multivibrator units 11ti(b-k) identical to the multivibrator units Z9 of the counter circuit 81 except that a multivibrator unit corresponding to the multivibrator unit 81a of counter -81 is omitted. Thus, ten multivibrator units llMb-k) are provided. The ten such multivibrator units 1MM-k) are connected to the last ten units 8Mb-k) of the counter 81 so that a transfer l.of the net count in the counter 81 is effectively divided by a factor o-f two thereby providing an average net count in the transfer circuit `36.

In the transfer circuit S6 as shown in FIG. 6B, each of the multivibrator units has transistors 99a and 91a. Transistors 90a and 91a are each provided with a transfer switch 111 and 112 in the `form of a transistor. The transistors of the switches 111 and 112 have their collectors connected to the respective collectors of the multivibrator transistor 96a and 91a and the respective emitters of the transistors in switches 111 and 112 are connected to a common input 113. Input 113 is common to all of the multivibrators 11tl(bk) and has input terminals 113@ and 113k where terminal 113a is connected to switches 111, 112 and terminal 11319 is connected by a resistance `115 to ground. The bases of the transistors in the respective transfer switches 111, 112 are respectively coupled by conductors 1&5, 106 to the outputs of a corresponding positioned multivibrator in the counter 81. In operation, an input or transfer signal 87a from Cil delay circuit 87 (after the net count is in counter 81) is supplied to input 113 which causes the transistors in the transfer switches 111, 112 of each multivibrator unit 110(b-k) to conduct in preference to the conductive state of the corresponding transistor 9i) or 91 in the counter 81 so that the respective transistors 96a and 91a in each multivibrator unit INQ7-k) align their respective conductive states to` correspond to the respective conductive states of transistors 90 and 91 in the counter S1. Thus, the net count in the last ten units of counter S1 is transferred to the multivibrator units of the transfer circuit 86. An output conductor 118 coupled to the second transistor of each of the multivibrator units 110(bk) and connected to the BAC circuit 89 which converts the digital number of the multivibrator units 11tl(b-k) into an `analog voltage signal in a welhknown manner.

It should be appreciated that the number of multivibrator units of counter S1 may be less than the number required to count the entirety of pulses from an add input provided that the number of units are adequate to ind-icate the difference between an add and subtract inputs.

It should also be appreciated that the described timed sequence of signals between the transmitter and receivers S T1R2-T1R1+T2R1T2R2 This sequence may, for example, be changed by reversing the output connections B and C to the input connections B and C of the borehole instrument 10. Assuming that B (not shown) is the output of iiip-iiop 27 connected to input connector C in the instrument 10 and that C' (not shown) is the output of the second stage flip-flop 28 connected to input connector B in the instrument 10; then as shown in FIG. 9, the counter S1 would measure a sequence as follows:

In the case of FIG. 8, the net count remaining in the counter 81 even though .represented by a negative number is still representative of the travel time.

In still another example, of the first stage `of flip-flop 27 rather than the second stage Iis connected to input connector C in the instrument 10 by a conductor B (not shown) and a conductor C (not shown) is the output of the second stage `of flip-flop 2S connected to input connector B in the instrument 10; then, as shown in FIG. l0, the counter `81 would measure a sequence as follows:

T 2R1- TiRi-l- TiRz- TzRz In this case the net count remaining in the counter would still be representative of the travel time.

Other sequences are `also possible provided that the main objective of subtracting the lesser time measurements Ifrom the greater time measurements is accomplished. This is made possible because only the net count in the counter is read out. Thus, the sequence of entering the time measurements may be changed without affectng the net count provided that the addition and subtraction is properly related to obtain the average time At.

t will also be appreciated that while the average net count is obtained by dividing the count in counter 81 by a particular relationship between the counter S1 and transfer circuit S6, other Well-known arrangements may be used to divide the total count in the counter 81 by a factor of two.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and m-odications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

l. An acoustic logging system for use in a Well bore comprising: a borehole instrument sized for passage through a well bore and having spaced upper and lower acoustic transmitter means and at least two acoustic receiver means therebetween, means coupled to said transmitter and receiver means for developing signals representing transmitter-to-receiver acoustic travel times for successive depth intervals between each of said transmitter means and each of said receiver means; means coupled to said signal developing means to provide counter pulse signals in response to said developed signals, counter means coupled to said counter pulse signal means for adding and subtracting at least two pulse signals from said counter pulse means representing at least two transmitter-to-receiver acoustic travel times for a given depth in a well bore to provide a net pulse count, transfer means coupled to said counter means and responsive to a transfer signal to read out a net pulse count in said counter means, means to key said counter means, said transfer means and said signal developing means in sequence to obtain an addition and subtraction of at least two pulse signals representing transmitter-toreceiver acoustic travel times for a given depth in a well fbore and provide a transfer signal Ito said transfer means to transfer 4such net pulse count in said counter means to said transfer means, and means coupled to said transfer means to derive indica- .tions of the net pulse count in the counter at the time of occurrence of a transfer signal.

2. An acoustic logging system comprising: a borehole instrument sized for passage through a well bore and having an upper and lower acoustic transmitter means, and upper and lower receiver means therebetween, signal channel means in said instrument, transmitter actuating means in said instrument including an output to said signal channel means, receiver switch means coupled between said receiver means and said signal channel means for selectively connecting one or the other of said receiver means to said signal channel means, transmitter switch means coupled between said transmitter means and said transmitter actuating means for selectively connecting one or the other of said transmitter means to said transmitter actuating means; surface located means coupled to said switch means and said transmitter actuating means for keying the operation of said switch means and said transmitter actuating means in a repetitive sequence to connect one of said transmitter means to said transmitter actuating means once for each connection of the respective receiver means to the signal channel means and the other of said transmitter means to said transmitter actuating means once for each connection of the respective receiver means to the signal channel means, means coupled to said signal channel means to provide counter pulse signals in response to signals representing transmitter-to-receiver acoustic travel time, counter means coupled to said counter pulse signal means for adding and subtracting pulse signals from said counter pulse means to provide a net pulse count, transfer means coupled to said counter means and responsive to a transfer signal to read out a net pulse count in said counter means, means coupled to said keying means to actuate said counter means and transfer means in sequence to obtain an addition and subtraction of at least two pulse signals representing transmitter-to-receiver travel times for a given depth in a well bore and provide a transfer signal to said transfer means to transfer such net pulse count in said counter means to said transfer means, and means coupled to said transfer means to derive indications of the net pulse count in the counter at the time of occurence of a transfer signal.

3. In an acoustic logging system for use in a well bore wherein it is desired to measure the difference between at least two independently derived time interval signals which are derived for successive depth intervals along a well bore, means to provide counter pulse signals in response to time interval signals, counter means coupled to said counter pulse signal means for adding and subtracting at least two pulse signals from said counter pulse means representing at least two time interval signals for a given depth in a well bore to provide a net pulse count,

transfer means coupled to said counter means and responsive to a transfer signal to read out a net pulse count in said counter means, means to key said counter means and transfer means in sequence to .obtain an addition and subtraction of at least two pulse signals representing time interval signals for a given depth in a well bore and provide a transfer signal to said transfer means to transfer such net pulse count in said counter means to said transfer means, and means coupled to said transfer means to derive indications of the net pulse count in the counter at the time of occurrence of a transfer signal.

4. In an acoustic logging system for use in a well bore wherein it is desired to measure the difference between at least two independently derived time Ainterval signals which are derived for successive depth intervals along a well bore, means to provide counter pulse signals in response to time interval signals, digital counter means coupled to said counter pulse signal means for adding and subtracting at least two pulse signals from said counter pulse means representing at least two time interval signals for a given depth in a well bore to provide a net pulse count, digital transfer means coupled to said counter means and responsive to a transfer signal to read out a net pulse count in said counter means, means to key said counter means and transfer means in sequence to obtain an addition and subtraction of at least two pulse signals representing time interval signals for a given depth in a well bore and provide a transfer signal to said transfer means to transfer such net pulse count in said counter means to said transfer means, and means coupled to said transfer means to derive indications of the net pulse count in the counter at the time of occurrence of a transfer signal including a digital-to-analog converter means and recorder means.

5. In an acoustic logging system for use in a well bore wherein it is desired to measure the difference between at :least =two independently derived time interval signals for successive depths along a well bore, a gate circuit operative in response to time interval signals for passing pulse signals therethrough during the time interval of such time interval signals, a idigital counter circuit coupled to the output of said gate circuit and a clock oscillator to provide pulse signals coupled to the input of said gate circuit, said counter circuit including add means to condition said counter circuit to add a pulse signal from said clock oscillator during one time interval when said gate is operative and subtract means to condition said counter circuit to subtract a pulse signal from said clock oscillator during another time interval when said gate is operative thereby to provide a net pulse count in said counter means, digital transfer circuit means coupled to said counter means and responsive to a transfer signal to read out a net count of pulse signals in said counter circuit representing the difference between added and subtracted pulse signals in said counter, means to key the operation of said add, subtract and transfer circuit means in a se- 'lected sequence to obtain an addition and subtraction of pulse signals, and a transfer of a net pulse count, circuit means coupled to said transfer circuit means for converting a net pulse count in said transfer circuit means into an analog signal, and means coupled to said converting circuit means for indicating the time difference between added and substracted time intervals as represented by such analog signal for successive depth intervals in a well bore.

6. An acoustic logging system for use in a well bore comprising: a borehole instrument sized for passage through a Iwell bore and having spaced upper and lower acoustic transmitter means and at least two acoustic receiver means therebetween, means coupled to said transmitter and receiver means `for developing signals representing transmitter-to-receiver,acoustic travel times for successive depth intervals between each of said transmitter means and each of said receiver means; means coupled to said signal developing means to provide 13 counter pulse signals in response to said Vdeveloped signals, counter means coupled to said counter pulse signal means for adding and subtracting at least two pulse signals from said counter pulse means representing at least two transmitter-to-receiver acoustic travel times for a given depth in a well bore to provide a net pulse count, said counter means being comprised of digital counting units, transfer means coupled to said counter means and responsive to a transfer signal to read out a net pulse count in said counter means, said transfer means being comprised of digital counting units, said transfer ldigital counting units being 14 coupled to said counter digital counting units beginning with the second of said counter digital counting units thereby to divide the net count in said counter means by a factor of two.

7. The apparatus of claim 6 wherein the number of said counter ydigital counting units is great enough to count the net difference between at least two pulse signals from said counter pulse means but less than the number of counter digital counting units required to count the 10 greatest pulse signal from said counter pulse means.

No references cited. 

1. AN ACOUSTIC LOGGING SYSTEM FOR USE IN A WELL BORE COMPRISING: A BOREHOLE INSTRUMENT SIZED FOR PASSAGE THROUGH A WELL BORE AND HAVING SPACED UPPER AND LOWER ACOUSTIC TRANSMITTER MEANS AND AT LEAST TWO ACOUSTIC RECEIVER MEANS THEREBETWEEN, MEANS COUPLED TO SAID TRANSMITTER AND RECEIVER MEANS FOR DEVELOPING SIGNALS REPRESENTING TRANSMITTER-TO-RECEIVER ACOUSTIC TRAVEL TIMES FOR SUCCESSIVE DEPTH INTERVALS BETWEEN EACH OF SAID TRANSMITTER MEANS AND EACH OF SAID RECEICER MEANS; MEANS COUPLED TO SAID SIGNAL DEVELOPING MEANS TO PROVIDE COUNTER PULSE SIGNALS IN RESPONSE TO SAID DEVELOPED SIGNALS, COUNTER MEANS COUPLED TO SAID COUNTER PULSE SIGNAL MEANS FOR ADDING AND SUBTRACTING AT LEAST TWO PULSE SIGNALS FROM SAID COUNTER PULSE MEANS REPRESENTING AT LEAST TWO TRANSMITTER-TO-RECEIVER ACOUSTIC TRAVEL TIMES FOR A GIVEN DEPTH IN A WELL BORE TO PROVIDE A NET PULSE COUNT, TRANSFER MEANS COUPLED TO SAID COUNTER MEANS AND RESPONSIVE TO A TRANSFER SIGNAL TO READ OUT A NET PULSE COUNT IN SAID COUNTER MEANS, MEANS TO KEY SAID COUNTER MEANS, SAID TRANSFER MEANS AND SAID SIGNAL DEVELOPING MEANS IN SEQUENCE TO OBTAIN AN ADDITION AND SUBTRACTION OF AT LEAST TWO PULSE SIGNALS REPRESENTING TRANSMITTER-TO-RECEIVER ACOUSTIC TRAVEL TIMES FOR A GIVEN DEPTH IN A WELL BORE AND PROVIDE A TRANSFER SIGNAL TO SAID TRANSFER MEANS TO TRANSFER SUCH NET PULSE COUNT IN SAID COUNTER MEANS TO SAID TRANSFER MEANS, AND MEANS COUPLED TO SAID TRANSFER MEANS TO DERIVE INDICATIONS OF THE NET PULSE COUNT IN THE COUNTER AT THE TIME OF OCCURRENCE OF A TRANSFER SIGNAL. 